Several key reactions present in Streptococcus mutans have been examined using mutant analysis techniques. Two enzyme systems for ammonia assimilation have been identified: glutamate dehydrogenase and the coupled reactions of glutamine synthetase and glutamate synthase. The primary pathway for sucrose utilization has also been determined. Catabolism of sucrose was found to be initiated by a highly efficient phosphoenolpyruvate-dependent sucrose phosphotransferase reaction followed by hydrolysis of sucrose phosphate by an intracellular sucrose phosphate hydrolase. In addition, the regulation and function of ethanol dehydrogenase was examined. This fermentation enzyme was found to be required for metabolism of mannitol and sorbitol.